1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using an ion doping method, and more specifically to a method of forming a protective film as a pretreatment in an ion doping step. In this specification, the “semiconductor device” denotes any of semiconductor devices in general which has a circuit structure with a thin film transistor (hereinafter abbreviated as a “TFT”), and semiconductor display device such as an active matrix type liquid crystal display device, an organic electro-luminescence (EL) display device, or the like are included in this category.
2. Description of the Related Art
Recently, demands for active matrix type liquid crystal display devices have increased rapidly and development of the technique for manufacturing TFTs with a semiconductor film formed on a glass substrate or a quartz substrate has been carried out actively. TFTs manufactured on an insulating substrate such as a glass substrate or the like in a unit of one million and several hundreds of thousands of pieces have to exhibit predetermined electric characteristics according to the function of an electric circuit to be formed therewith. There is a parameter called “Vth” as one of the electric characteristics of a TFT.
The “Vth” denotes a gate voltage measured at the moment when a drain current of a TFT starts flowing and is defined as a voltage at which an inversion layer is formed in a channel region. Hence, it can be said that the higher the Vth is, the higher the TFT operating voltage is.
Note that, the Vth has a problem in that it fluctuates easily by various external factors including, for instance, contamination impurities in an active layer, fixed and mobile charges in a gate insulating film, an interface level at an active layer/gate insulating film interface, and the difference in work function between a gate electrode and an active layer. In this case, the contamination impurities in an active layer, the mobile charge in a gate insulating film, and the like can be reduced through cleaning in processes. However, the fixed charge, the interface level, and the difference in work function depend on the device material and thus cannot be modified easily.
The above-mentioned external factors cause the Vth of a TFT to shift to the plus or minus side to vary. In TFT manufacturing steps, control of variable Vth is an important technique, and a channel doping technique has been known as a Vth control technique. The “channel doping” is a technique for controlling Vth by adding a predetermined concentration of impurity to an active layer located under a gate insulating film to shift the Vth of a TFT intentionally so that the Vth reaches a desired level. For example, a p-type impurity element is used as a dopant when the Vth shifts to the minus side, while an n-type impurity element is used as a dopant when the Vth shifts to the plus side. Thus, the Vth is controlled.
For such channel doping, an ion doping method for doping with an n-type or p-type impurity element is used. The ion doping method is a method of implanting an impurity element without implementing mass separation. Since the ion doping method employs no mass separation means, it is easy to achieve an increase in area subjected to processing. Hence, the ion doping method is generally applied to the manufacture of an active matrix type liquid crystal display device. In the ion doping method, B(boron), Ga(gallium), or In(indium) is used as a p-type impurity, and P(phosphorus), As(arsenic), Sb(antimony), or the like is used as an n-type impurity.
When a doping process such as channel doping or the like is carried out directly with respect to a silicon-based semiconductor film as an active layer of a TFT, there is a problem in that the silicon-based semiconductor film is etched. Conventionally, as measures for solving the problem, a protective film such as a silicon oxide film, a silicon oxynitride film, or the like is deposited by a chemical vapor deposition (CVD) method as a pretreatment in a doping step and then the process of doping with impurity ions is conducted from the top of the protective film. However, the measures have the following demerits and therefore are not preferable.
First, since the CVD method is applied to the mere pretreatment, the time required for the pretreatment is lengthened and the processing time required for the whole step of doping with impurity ions is also lengthened accordingly. Therefore, with respect to the whole step of doping with impurity ions, the above-mentioned measures are not preferable in terms of throughput since the number of substrates to be processed per unit time is reduced. In addition, the above-mentioned measures also are not preferable in view of the fact that the cost for the pretreatment increases since a CVD apparatus such as a plasma CVD apparatus, a low pressure CVD apparatus, or the like is used for the pretreatment and thus the whole production cost increases accordingly. Therefore, an easy low-cost measure for preventing etching has been requested as a measure for preventing a silicon-based semiconductor film from being etched.